Characterization investigations of ZrB2/ZrC ceramic powders synthesized by mechanical alloying of elemental Zr,B and C blends

ZrB₂/ZrC ceramic powders were fabricated by mechanical alloying (MA) of zirconium (Zr), amorphous boron (B) and graphite (C) powder blends prepared in the mole ratios of Zr/B/C: 1/1/1, 1/2/1, 1/1/2, 1/2/2 and 2/2/1. MA runs were carried out in a vibratory ball mill using hardened steel vial/balls. The effects of Zr/B/C mole ratios and milling duration on the formation and microstructure of ZrB₂/ZrC ceramic powders were examined. Gibbs free energy change-temperature relations of the reactions and moles of the products were interpreted by thermochemical software. Zr/B/C: 1/1/1, 1/2/1, 1/1/2 and 1/2/2 powder blends MA’d for 2 and 3 h contain unreacted Zr and C, ZrB₂, ZrC and B₄C particles. Synthesis of ZrB₂/ZrC ceramic powders was completely accomplished after MA of Zr/B/C: 2/2/1 powder blend for 2 h. ZrC and ZrB₂ particles were obtained ranging in size between 50 and 250 nm in the presence of FeB contamination (<1 wt.%).     

Structural Influence on the Thermal Conversion of Self‐Catalyzed HfB2/ZrB2 Sol–Gel Precursors by Rapid Ultrasonication of Oxychloride Hydrates

Sol–gel precursors to HfB₂ and ZrB₂ are processed by high‐energy ultrasonication of Hf,Zr oxychloride hydrates, triethyl borate, and phenolic resin to form precipitate‐free sols that turn into stable gels with no catalyst addition. Both precursor concentration and structure (a sol or a gel) are found to influence the synthesis of the diboride phase at high temperature. Decreasing sol concentration increases powder surface area from 3.6 to 6.8 m²/g, whereas heat‐treating a gel leads to residual oxides and carbides. Particles are either fine spherical particles, unique elongated rods, and/or platelets, indicating particle growth with directional coarsening. Investigation of the conversion process to ZrB₂ indicates that a multistep reaction is likely taking place with: (1) ZrC formation, (2) ZrC reacts with B₂O₃ or ZrC reacts with B₂O₃ and C to form ZrB₂. At low temperatures, ZrC formation is limiting, while at higher temperatures the reaction of ZrC to ZrB₂ becomes rate limiting. ZrC is found to be a direct reducing agent for B₂O₃ at low temperature (~1200°C) to form ZrB₂ and ZrO₂, whereas at high temperatures (~1500°C) it reacts with B₂O₃ and C to form pure ZrB₂.     

C/C–ZrC composite prepared by chemical vapor infiltration combined with alloyed reactive melt infiltration

A high performance and low cost C/C–ZrC composite was prepared by chemical vapor infiltration combined with zirconium–silicon (Zr: 91.2 at.%; Si: 8.8 at.%) alloyed reactive melt infiltration. The density of the as-received composite is 2.46 g/cm³ and the open porosity is 5%. Due to the reaction between the pyrolytic carbon and Zr–Si alloy in the composite, ZrC and Zr₂Si phases were formed, the formation and distribution of which were investigated by thermodynamics and phase diagram. The as-received C/C–ZrC composite, with the flexural strength of 239.5 MPa, displayed a pseudo-ductile fracture behavior. Ablation properties of the C/C–ZrC composite were tested by a pulse laser. The linear ablation rate was 0.028 mm/s. A ZrO₂ barrier layer was formed on the ablation surface and the composite presented excellent ablation resistance.     

Reaction Mechanism and Microstructure Development of Zr–Si Alloyed Melt‐Infiltrated ZrC‐Modified C/C Composite

ZrC ceramic modified‐C/C composite is prepared by a quick and low‐cost reactive melt infiltration process with a Zr‐Si8.8 alloy. Reaction kinetics and mechanism of pyrolytic carbon with the infiltrated Zr‐Si8.8 alloy are investigated. A continuous ZrC layer is found to be formed around pyrolytic carbon due to the in situ reaction and its thickness parabolically increases with an increase in reaction time period. Zr concentration in the alloyed melts decreases due to the reaction between Zr and pyrolytic carbon and Zr₂Si phase precipitates from the residual alloyed melts. A model for the growth of ZrC layer is established to describe the reaction kinetics of pyrolytic carbon with Zr‐Si8.8 alloy. The calculated thickness of the reaction‐formed ZrC layer is in good agreement with the experimental data. Based on the Arrhenius equation, the activation energy of the reaction between carbon and Zr‐Si8.8 alloy is 313.2 KJ/mol, smaller than that of the reaction between carbon and pure zirconium. The microstructure of the reactive melt‐infiltrated ZrC‐modified C/C composite is characterized by optical microscope, SEM, EDS, XRD, and TEM. The mechanism of the reaction between pyrolytic carbon and Zr‐Si8.8 alloy is discussed on the basis of the characterization results and phase diagram. A schematic is proposed to understand the reaction mechanism between pyrolytic carbon and Zr‐Si8.8 alloy and microstructure development of the ZrC‐modified C/C composite.     

Spark Plasma Sintering of Superhard B4C–ZrB2 Ceramics by Carbide Boronizing

A carbide boronizing method was first developed to produce dense boron carbide‐ zirconium diboride (“B₄C”–ZrB₂) composites from zirconium carbide (ZrC) and amorphous boron powders (B) by Spark Plasma Sintering at 1800°C–2000°C. The stoichiometry of “B₄C” could be tailored by changing initial boron content, which also has an influence on the processing. The self‐propagating high‐temperature synthesis could be ignited by 1 mol ZrC and 6 mol B at around 1240°C, whereas it was suppressed at a level of 10 mol B. B₈C–ZrB₂ ceramics sintered at 1800°C with 1 mole ZrC and 10 mole B exhibited super high hardness (40.36 GPa at 2.94 N and 33.4 GPa at 9.8 N). The primary reason for the unusual high hardness of B₈C–ZrB₂ ceramics was considered to be the formation of nano‐sized ZrB₂ grains.     

Synthesis of ZrB2–ZrC hybrid powders via boro-carbothermal reduction of ZrO2 by B4C and carbon black

ZrB₂–ZrC hybrid powders were synthesized by a novel two-step reduction on basis of ZrO₂ + B₄C + C→ ZrC + ZrB₂ + CO reaction in Ar atmosphere, using ZrO₂, B₄C, and carbon black powders as starting materials. Thermodynamics of relevant reactions were evaluated. Effects of excess additions of B₄C and C on phase constituents were investigated. Morphology and chemistry of the powder products were characterized by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and transmission electron microscopy (TEM). The results showed that ZrB₂–ZrC hybrid powders with no obvious impurity content could be obtained after heating at 1350 °C for 1 h followed by further reaction at 1700 °C for 1 h with 16 wt% B₄C + 8 wt% C excess addition. Relative contents of the ZrB₂: ZrC phase in the product powders could be conveniently regulated by varying the B₄C and C content in the starting compositions. The resultant powders had good oxidation resistance with an oxidation activation energy value of 433 kJ/mol. Good sinterability of the powder products was demonstrated by hot pressing at 1950 °C for 60min under 30 MPa pressure, which resulted in fully dense ZrB₂–ZrC composite ceramics with Vickers hardness value larger than 18.3 ± 0.6 GPa.     

Densification and toughening mechanisms in spark plasma sintered ZrB2-based composites with zirconium and graphite additives

The densification behavior and toughening mechanisms of ZrB₂-based composites with in-situ formed ZrC were investigated. The composites were spark plasma sintered at 1700 °C for 7 min under the applied pressure of 40 MPa. Metallic zirconium and graphite flakes were used as precursors to achieve ZrC reinforcement. Microstructural and phase analyses as well as mechanical characterizations were carried out on the near fully-dense composite samples. Results indicated ZrC as the only secondary phase in composite with 5 vol% of metallic Zr and graphite flakes. However, higher volume fractions of precursor materials led to the formation of ZrO₂ as the dominant secondary phase. Whereas decreasing trend of the hardness number versus volume fraction of the precursors was observed, the highest indentation fracture toughness was achieved in sample with 15 vol% metallic Zr/graphite flakes. Finally, the formation of secondary phases and their effects on densification, and mechanical behavior of the composites were discussed.     

A computational analysis of a ZrO2–SiO2 scale for a ZrB2–ZrC–Zr ultrahigh temperature ceramic composite system

The success of a ceramic composite for ultrahigh temperatures (i.e., >1873 K) in an oxidizing atmosphere resides in the protective characteristics of a scale to limit oxygen ingress or to control the oxygen reaction into the substrate. With temperature changes from room temperature to ultrahigh temperatures, the mechanics of the scale and its reactivity becomes critical for ceramic composites to operate under extreme environments. A study was pursued to design computationally a SiO₂–ZrO₂ scale for a ZrB₂/ZrC/Zr–Si composite by using conventional finite element analysis, which was used as a baseline microstructure for the extended finite element method. The model of the Zr boride/carbide composite with a SiO₂/ZrO₂/ZrSi_x scale simulates the development of local strain energetics under a thermal load from 300 to 1700 K. The computational analysis determined that the size of the SiO₂ and ZrSi_x precipitates does not appreciably influence the durability of the microstructure. A simulated annealing optimization algorithm was also developed for an extended finite element program (called XMicro) with the purpose of optimizing the auto re-meshing of XMicro and thus minimizing its combinatorial selection of a composite's reinforcement architecture. After correcting for the overlapping of ZrO₂ precipitates within a matrix, XMicro determined that 1.96 μm as the optimal spacing of precipitates within a cluster and 20 μm between clusters within a silica matrix of the scale interphase. The strategic experimentation determined that porosity developed during oxidation should be incorporated into the simulation of a ceramic composite. To probe into the efficacy of the silica layer for the scale, oxidizing experiments were performed at 1973 K, as well as microstructural analysis of the scale interphase. The computational mechanics coupled with consideration of the thermodynamic stability of phases for the Zr–Si–O system to set the oxygen potentials between layers can design a scale interphase for an ultrahigh-temperature, ceramic composite system. The processing challenge would be to attain the optimal configuration of the microstructure, for example, silicide precipitates developed with the appropriate spacing along a scale/matrix interface or ZrO₂ clusters within a silicate phase.     

Chemical process and solid solution formation in reactive hot pressed ZrB2–SiC ceramics doped with W

ZrB₂–SiC doped with W was prepared from a mixture of Zr, Si, B₄C and W via reactive hot pressing. The fully dense ZrB₂–SiC–WB–ZrC ceramic was obtained at 1900°C for 60 min under 30?MPa in an argon atmosphere. Reaction path and solid solution characteristics of the starting powders were studied through a series of pressureless heat treatment at temperatures between 700 and 1500°C. The solid solution phases of (Zr, W)B₂, (W, Zr)B and (Zr, W)C were formed directly by reactions between the precursors. Homogeneous distribution of solute atoms in solution and the solid solubilities were also studied.     

Fabrication and properties of reactively hot pressed ZrB2–SiC ceramics

ZrB₂–SiC ceramics with relative densities >99% were fabricated by ‘in situ’ reactive hot pressing from ZrH₂, B₄C and Si. The reaction was studied using two processes, (1) powder reactions at temperatures from 1150 to 1400 °C and (2) reactive hot pressing between 1600 and 1900 °C. The products from the reaction of a 2ZrH₂:1B₄C:1Si molar mixture were ZrB₂, SiC, ZrO₂ and ZrC. Modification of the composition to 2ZrH₂:1.07B₄C:1.16Si resulted in the elimination of the undesired ZrO₂ and ZrC phases. The final composition was approximately ZrB₂–27 vol% SiC with no undesired phases detected by X-ray diffraction, and only low concentrations of B₄C detected by scanning electron microscopy. Elimination of the undesired phases was accomplished by removing surface oxides through chemical reactions at elevated temperatures. Reactively hot pressed samples consisting of ZrB₂ with 27 vol% SiC had a Young's modulus of 508 GPa, a flexure strength of 720 MPa, a fracture toughness of 3.5 MPa m^1/2 and a Vickers’ hardness of 22.8 GPa.     

Reactive Hot Pressing and Properties of Zr1−xTixB2–ZrC Composites

Reactive hot pressing was used to prepare Zr_1?xTi_xB₂–ZrC composites with advantageous microstructure and mechanical properties from ZrB₂–TiC powders. The reaction mechanisms and the effects of different levels of TiC on the physical and mechanical properties of the resulting composite were explored in detail and compared to conventionally hot‐pressed ZrB₂ and ZrB₂–ZrC. Incorporation of 10 to 30 vol% TiC enabled full densification and restrained grain growth, reducing the final average grain size from 5.6 μm in pure ZrB₂ to a minimum of 1.4 μm in samples with 30 vol% TiC. The flexural strengths and hardnesses of the composites sintered with TiC were consequently greater than the conventionally processed ZrB₂–ZrC materials, increasing from 440 MPa and 17.4 GPa to a maximum of 670 MPa and 24.2 GPa at 10 vol% TiC. However, despite a decrease in the total average grain size, the flexural strength at higher TiC levels was limited by an increase in ZrC grain growth, which was observed to determine the flexural strength of the reaction sintered composites similar to the case of ZrB₂–SiC.     

Structural characteristics and ablative behavior of C/C–ZrC–SiC composites reinforced with “Z-pins like” Zr–Si–B–C multiphase ceramic rods

In order to improve the ablation and oxidation resistance of C/C–ZrC–SiC composites in wide temperature domain, “Z-pins like” Zr–Si–B–C multiphase ceramic rods are prepared in the matrix. The influence of different sintering temperatures on the microstructure of ceramic rods and the ablative behavior of heterogeneous composites are studied. The results showed that the ZrB₂ and SiC phases are formed in the sintered matrix, and the increase of sintering temperature is beneficial to improve the density of the ceramic rods. The ablation properties of samples have been greatly improved. The mass and linear ablation rate are 0.8 mg/s and 3.85 μm/s, respectively, at an ablation temperature of 3000 °C and an ablation time of 60 s. After ablation, the matrix surface is covered with SiO₂ and ZrO₂ mixed oxide films. This is due to the preferential oxidation of “Z-pins like” Zr–Si–B–C multiphase ceramic rods in the ablation process, and B₂O₃ melt, SiO₂ melt, borosilicate glass, ZrSiO₄ melt and ZrO₂ oxide film can be generated successively from the low-temperature segment to the ultra-high temperature segment. These oxidation products can be used as compensation oxide melts for the healing of cracks and holes on the matrix surface in different temperature ranges and effectively prevent the external heat from spreading into the matrix. Therefore, C/C–ZrC–SiC composites with “Z-pins like” Zr–Si–B–C multiphase ceramic rods achieve ablation resistance in wide temperature domain.     

Contact interaction and hot pressing of ZrB2-MoSi2 in CO/CO2 atmosphere

ZrB₂-15 vol% MoSi₂ ceramics were hot pressed in CO/CO₂ atmosphere in the 1700–1900^oС temperature range. During hot pressing, MoSi₂ decomposes into Mo and Si and the phase composition of the as-sintered ceramic results in ZrB₂, (Zr, Mo)B₂, SiC, SiO₂, and MoB. Contact melting between ZrB₂ and MoSi₂ was observed at 1800^oC, corresponding to the formation of (Zr, Mo)B₂. Ceramics obtained at1800–1850^oС had ∼ 500 МPа and 200 MPa strength at room at 1800^oC in vacuum, respectively. The thickness of the oxidized scales upon exposing the samples at 1600 ^oC for 120 min was 30–80 µm and depended on the amount of residual MoSi₂ and (Zr, Mo)B₂. The highest oxidation resistance was observed for the ceramic sintered at 1850 °C.     

Reactive Hot Pressing of ZrC–SiC Ceramics at Low Temperature

Composites of ZrC–SiC with relative densities in excess of 98% were prepared by reactive hot pressing of ZrC and Si at temperature as low as 1600°C. The reaction between ZrC and Si resulted in the formation of ZrC_1?x, SiC, and ZrSi. Low‐temperature densification of ZrC?SiC ceramics is attributed to the formed nonstoichiometric ZrC_1?x and Zr–Si liquid phase. Adding 5 wt% Si to ZrC, the three‐point bending strength of formed ZrC_0.8–13.4 vol%SiC ceramics reached 819 ± 102 MPa with hardness and toughness being 20.5 GPa and 3.3 MPa·m^1/2, respectively.     

Fabrication and mechanisms of densification of ZrB2-based ultra high temperature ceramics by reactive hot pressing

Dense ZrB₂–SiC (25–30 vol%) composites have been produced by reactive hot pressing using stoichiometric Zr, B₄C, C and Si powder mixtures with and without Ni addition at 40 MPa, 1600 °C for 60 min. Nickel, a common additive to promote densification, is shown not to be essential; the presence of an ultra-fine microstructure containing a transient plastic ZrC phase is suggested to play a key role at low temperatures, while a transient liquid phase may be responsible at temperatures above 1350 °C. Hot Pressing of non-stoichiometric mixture of Zr, B₄C and Si at 40 MPa, 1600 °C for 30 min resulted in ZrB₂–ZrC_x–SiC (15 vol%) composites of 98% RD.     

In Situ Synthesis of a ZrB2‐Based Composite Powder Using a Mechanochemical Reaction for the Zircon/Magnesium/Boron Oxide/Graphite System

A ZrSiO₄/B₂O₃/Mg/C system was used to synthesize a ZrB₂‐based composite through a high‐energy ball milling process. As a result of the milling process, a mechanically induced self‐sustaining reaction (MSR) was achieved in this system. A composite powder of ZrB₂–SiC–ZrC was prepared in situ by a magnesiothermic reduction with an ignition time of approximately 6 min. The mechanism for the formation of the product was investigated by studying the relevant subreactions, the stoichiometric amount of B₂O₃, and thermal analysis.     

Reactive sintering behavior and enhanced densification of (Ti,Zr)B2–(Zr,Ti)C composites

Reactive hot pressing was used to prepare (Ti,Zr)B₂–(Zr,Ti)C composites from equimolar ZrB₂ and TiC powders. The reaction and solid-solution coupling effect and enhanced densification in ZrB₂-50 mol.% TiC were proposed as contrasted to conventional consolidation of TiB₂-50 mol.% ZrC. The (Ti,Zr)B₂–(Zr,Ti)C composite sintered at a temperature as low as 1750 °C exhibited negligible porosity and average grain sizes of 0.30 μm for (Ti,Zr)B₂ and 0.36 μm for (Zr,Ti)C. Complete reaction and rapid densification of ZrB₂-50 mol.% TiC was achieved at 1800 °C for only 10 min. The densification mechanism was mainly attributed to material transport through lattice diffusion of Ti and Zr atoms with an activation energy of 531 ± 16 kJ/mol. This study revealed for the first time novel insights into rapid densification of refractory fine-grained diboride–carbide composites by reactive hot pressing at relatively low temperatures.     

ZrB2–SiC–G Composite Prepared by Spark Plasma Sintering of In‐Situ Synthesized ZrB2–SiC–C Composite Powders

To avoid introduction of milling media during ball‐milling process and ensure uniform distribution of SiC and graphite in ZrB₂ matrix, ultrafine ZrB₂–SiC–C composite powders were in‐situ synthesized using inorganic–organic hybrid precursors of Zr(OPr)₄, Si(OC₂H₅)₄, H₃BO₃, and excessive C₆H₁₄O₆ as source of zirconium, silicon, boron, and carbon, respectively. To inhabit grain growth, the ZrB₂–SiC–C composite powders were densified by spark plasma sintering (SPS) at 1950°C for 10 min with the heating rate of 100°C/min. The precursor powders were investigated by thermogravimetric analysis–differential scanning calorimetry and Fourier transform infrared spectroscopy. The ceramic powders were analyzed by X‐ray diffraction, X‐ray photoelectron spectroscopy, and scanning electron microscopy. The lamellar substance was found and determined as graphite nanosheet by scanning electron microscopy, Raman spectrum, and X‐ray diffraction. The SiC grains and graphite nanosheets distributed in ZrB₂ matrix uniformly and the grain sizes of ZrB₂ and SiC were about 5 μm and 2 μm, respectively. The carbon converted into graphite nanosheets under high temperature during the process of SPS. The presence of graphite nanosheets alters the load‐displacement curves in the fracture process of ZrB₂–SiC–G composite. A novel way was explored to prepare ZrB₂–SiC–G composite by SPS of in‐situ synthesized ZrB₂–SiC–C composite powders.     

Synthesis of ZrC–SiC Powders by a Preceramic Solution Route

Homogenous liquid precursor for ZrC–SiC was prepared by blending of Zr(OC₄H₉)₄ and Poly[(methylsilylene)acetylene]. This precursor could be cured at 250°C and converted into binary ZrC–SiC composite ceramics upon heat treatment at 1700°C. The pyrolysis mechanism and optimal molar ratio of the precursor were investigated by XRD. The morphology and elements analyses were conducted by SEM and corresponding energy‐dispersive spectrometer. The evolution of carbon during ceramization was studied by Raman spectroscopy. The results showed that the precursor samples heat treated at 900°C consisted of t‐ZrO₂ (main phase) and m‐ZrO₂ (minor phase). The higher temperature induced phase transformation and t‐ZrO₂ converted into m‐ZrO₂. Further heating led to the formation of ZrC and SiC due to the carbothermal reduction, and the ceramic sample changed from compact to porous due to the generation of carbon oxides. With the increasing molar ratios of C/Zr, the residual oxides in 1700°C ceramic samples converted into ZrC and almost pure ZrC–SiC composite ceramics could be obtained in ZS‐3 sample. The Zr, Si, and C elements were well distributed in the obtained ceramics powders and particles with a distribution of 100 ~ 300 nm consisted of well‐crystallized ZrC and SiC phases.     

In situ reaction and solid solution induced hardening in (Ti,Zr)B2-(Zr,Ti)C composites

(Ti,Zr)B₂ - (Zr,Ti)C ceramics were synthesized by reactive hot-pressing and solid solution coupling effect using ZrB₂ and TiC powders as starting materials. Effects of sintering temperature on phase relations, microstructure and mechanical properties were reported. The equimolar ZrB₂ and TiC reactants ensured a complete in situ reaction to form (Ti,Zr)B₂ and (Zr,Ti)C solid solutions. The (Ti,Zr)B₂ - (Zr,Ti)C composite sintered at 1750°C was fully densified, and exhibited a high hardness of 29.1 GPa due to fine-grain hardening and solid solution hardening. The optimized comprehensive mechanical properties such as a hardness of 27.9 GPa, a strength of 705 MPa and an indentation fracture toughness of 5.3 MPa m^1/2 were achieved in (Ti,Zr)B₂ - (Zr,Ti)C composites sintered at 1800°C for 1 hour.